Extraction of substances from solutions



v July 11, 1939. 1l w ALLQUlsT 2,165,438

EXTRACTIONQF SUBSTANCES FROM SOLUTIORS- l Filed July 25, 1936 INVENTORATTRNEYS Patented July 11, 1939 Y l Y l '2,165,438 `vi I MTE STATES.PATEN'TY'OFu-c Juhu william Ga., l

' '.lubilse Chatillon Corporation, NewYork, N. Y..

ai corporation of Delaware 'i application July 25, 193s, serial No.V92.528

comms. (ci. 26o-541) This invention relates to extraction ofsubemployedshould rhave bothlhigh gleanalillityf` stances fromsclunonsthereor and is` particuand selectivity, By skanabitWIxmean l:thelarly concerned with the extraction of water Vpower which Some SOlVeIltShave; t extract 011- soluble fatty acids, such as acetic acid, fromrglean practically all of the acetic acid from the l Vs aqueoussolutions. v solution so asto"leave it substantially .freesof. `I y Inmanufacturing rayon bythe acetate procacetic acid, althoughacompartvelylalge PTO- ess, large quantities of dilute aqueousA aceticacid DCFJOn 0f Water may also be diSSOlVed Aby the? .--v'

Solutions are produced. The acetic acid consclvent'fin gleaning theaceticfacidy- `Byselec tent of these solutions cannot be'returned totiVy I'jmean'the POWel t0 extract aCetiCfCid lo the process unless it isin concentrated form, from the solution while extractingllttle or noli)vIt is possible to concentrate the acetic acid .by water from thesolution. Unfortunately, no sin-2;. y. f directly distilling the diluteaqueous solutionof gle commercial solvent or.solventmi xtureposfy aceticacid. This, however, is not economically Sesses both-properties in anadequate degree. i1 i desirable because of the large heat consumption Ingeneral, solvents for acetic acid thatgare Y l involved. substantiallyimmlscibledn water have high vsel! l T0 avoid this high heat consumptionit has lectivity and low` gleanability, i. e.,jthey extract" a beenproposed heretofore to'employ a.- solvent acetic acid from aqueoussolutions without ex- !or acetic acid which will ,vextract the aceticacid tracting 'a rlatively 'large DIODOrtOn 0f Waterr from the water butis at least partially immiscible 'ibut they do not tend to leave thedepleted aquey .20 therewith, so thatasolutionwith a highencon-.fouslsolution free'of acetic acid unless-an ab- 20 centration of aceticfacidis produced and seanormally large proportion of solvent to solutionrated out. The/amount of heat necessary to .is employed. Among suchsolvents arebenzene, separate the acetic acid fromrvthe solvent in thispehtane and-iSOPrOPl/l ethelmore concentrated separated solution is`appre- Conversely, solvents which are relativelymis- Y 2.5 ciably lessthan the amount of heat necessary cible with water generally have highgleanability to separate the acetic yacid directly from the and lowselectivity; i. e., a relatively small quanwater. tity of the solventwill extract all the acetic ac ld In accord ce with the aforementionedprofrom the solution but in doing so will pick up posal thedil tesolution of acetic acid and the a relatively large quantity of water.Examples solvent pass countercu'rrent to each other of such solvents areethyl ether and ethylacetate. 30v through a tower or other container. Ifthe sol- The difference in theaction of these two classes i Y y ventAisof lower specific gravity'than the soluof s olventsis shown by thefollowing examples: tion it is admitted at the bottom o f the tower Ifisopropyl ether is used as a solvent extractor .and iloats upwardthrough` a pool of the solua nal solvent `productmay be obtained .conf'l f. a5 tion, extracting acetic acid (accompanied by taining 8% aceticacid and only 2.1% water, but 85 more or less water) on its way. Thedepleted inV obtaining suchfa product 'a large proportion ,Y

solution with little or no aceticV acid content of acetic acid mayremainin the solution, and,A is drained from Athe bottom. of the container.the ratio of the quantities of isopropyl ether to y The solvent whichhaspicked up acetic acid is vsolution may be high. i 1 v 40 drawn, fromthe top ofthe container, and sub- If, instead'of isopropyl ether, ethylacetate is 40 jected to treatment, such as distillationfto reemployedand the same procedure followed, a move its acetic acid content inconcentrated solvent product containing 12% acetic acidv and form. Ifthe solvent has a higher specific gravity 12% of water may be obtained,but a more comthan the solution the direction of llow is epleteextraction of acetic acid from the solution 45 versed-, i. e.. thesolvent is admitted at the top will `result with a lower ratio' ofsolvent to 45 ofthe chamber and sinksathrough a pool of the solution. y

solution, an enriched `solvent product being .,Ifa mixture of solventsselected from the-two drawn from the bottom and the depletedsolu-A'classes is employed the mixture does not have tion from the top of thecolumn.y the high gleanability of the miscible solvent and 5o In orderto obtain ideal results with such a the high selectivity of theimmiscible solvent. 50 process, namely, to obtain a residual aqueousSuch mixing merely results in averaging the varsolution containing noacetic acid and a solvent ious properties of the various solvents,whereas product containing a high concentration of acetic whatk is'desiredv is a process in which the high acid and littlev or no water(while employing a gleanability of the more miscible solvent and yrelatively small amount of solvent), the solvent the high selectivity ofthe lless miscible solvent 55 v extracting a large quantity of water.

cooperate in their effects to produce complete extraction of acetic acidfrom the solution'with a relatively small volume ofs'olvent, and asolvent product containing a h'igh concentration of acetic acid and a.low concentration of water.

Prior proposals to obtain 'cooperation of the high gleanability of arelatively miscible solvent with the high selectivity of a relativelyini-mis#4 cible solvent in extracting a fatty acid or the like from itsaqueous solution have included first mixing the solution with the moremiscible sl= vent, separating the depleted solution from the moremiscible solvent which has picked up the fatty acid, distilling thesolution of the fatty acid in the more miscible solvent to` obtain asecond aqueous solution of fatty acid of higher concentration, and thenmixing this second aqueous solution with the less miscible solvent. Theless miscible solvent, on account of the increased concentration of thefatty acid in the second 4aqueous solution, is enabled to extract arelatively large quantity of the fatty acid without 'I'here are variousobjections to this proposal, chief among which is the fact that thedistillation of the second aqueous solution involves extra handling andconsumes time and money.

It has also been proposed to bring an original aqueous solution of fattyacid into contact with a relatively misciblesolvent of highgleanability, and thus extract the fatty acid, separating the relativelymiscible solvent with the extracted fatty acid from the depletedsolution, and adding to and mixing with the separated miscible solventcontaining the fatty acid a second solvent of low miscibility and highselectivity.vv Upon mixing, water is dropped out of the mixture to someextent. Chiefobjections to this proposal arise from the fact that themixing of the two solvents merely produces a solvent mixture Yhaving theaverage gleanability and the average selectivity of the two solvents.VThis gleanability -is less than that of the first and relativelymiscible solvent so 'that some fatty acid is dropped out along with thewater. The selectivity of the mixture is also less than the selectivityof the second and less miscible solvent,so that the fatty acid retainedby the second solvent after mixing is accompanied by a greaterproportion of water than would accompany it if the original aqueoussolution had been treated directly with the second and less misciblesolvent.

As a result of my investigations I have invented a process whereby thehigh gleanability of a relatively miscible solvent and the highselectivity of a relatively immiscible solvent are enabled to cooperateto a degree not attained heretofore, with the result that athoroughextraction of a substance from a solution can be obtained with a minimumquantity of solvent while the solvents withdrawn from the system containa higher concentration of the extracted substance and a lowerconcentration of the original solvent.

My invention is based upon the discovery that. a column of liquid,through which the gleanability for the substance to be extractedgradually decreases as the selectivity for the substance increases, actsin a manner analogous to that in which a rectifying column operates in adistillation process. By maintaining a column of liquid containing suchan immiscible solvent, and by maintaining high selectivity in one zone.in the column and high gleanability in Aanother zone in the column,both the selectivity ,and the gleanabilityofth'e column as a whole areimproved. Thus, if the column contains two solvents, the first of highgleanability and low selectivity, the second of high selectivity and lowgleanability, the effective gleanability of the column as a wholeapproaches that of the first solvent while the effective selectivity ofthe column las a whole tends to approach the selectivity of the secondsolvent. In other words, the effective (gleanability of the column isgreater than the average gleanability of its two solvent constituentswhile the effective selectivity of the column is greater than theaverage selectivity of its two solvent constituents.

A solvent column of the character described may be maintained in severalways. Thus, one zone of the column may be maintained with highergleanability and lower selectivity than a second zone by keeping thefirst zone hotter than the second zone. A similar result may be obtainedby blending two solvents in the column, maintaining at one zone a highconcentration of a solvent having a high selectivity and a lowgleanability while maintaining at the other zone a high concentration ofa different solvent having a higher gleanability and a lowerselectivity.

'I'hese and otherl features of my invention will be more thoroughlyunderstood in the light of the following detailed description taken inconjunction with the accompanying drawing, in which- Fig. l is adiagrammatic representation of a conventional extractor used in theprior art.

Fig. 2 is a schematic representation of the solvent column which shouldbe maintained in the practice of my invention.

Fig. 3 is a diagrammatic representation of a presently preferred form ofextractor for the practice of my invention, and

Fig. 4 is a diagram of an apparatus adapted to a modified practice of myinvention.

Throughout the following description, the process is considered asconducted with solvents which have a lower specific gravity than theoriginal solution, so that when introduced into a column of the solutionthe solvents tend to rise to the top thereof. The process, of course,may be practiced also with solvents which are heavier than the originalsolutions, in which case the direction of flow of solution and solventsis reversed. l

The description is made with specific reference to extracting aceticacid from dilute aqueous solutions thereof. The process, however, is applicable to the extraction of many other substances from many othersolutions, because the principles involved are the same. In general, myprocess is applicable for extracting any soluble substance'from a dilutesolution thereof in a first solvent whenever there is available asolvent which has increased gleanability and decreased selectivity whenheated and which is at least partially immiscible with the firstsolvent, or whenever there are available two solvents for the substancewhich are at least partially immiscible with the first solvent, one ofthe two having high gleanability, the other high selectivity.

As used here, the general definition of gleanability is the power whichone solvent has to extract a solute from another solvent, leaving thedepleted liquid free of the solute to be recovered. The lower the'quantity of the slute in the de pleted liquid, the greater is thegleanability of tion process remaining constant.

.the highei will be the proportion of this other The general'definitionv of selectivity as fused here is the power of a solvent toextract a solute from a second solventwithout picking up a largeproportion of the second solvent as well.. The

greaterthe ratio of the quantity of solute extractedto thequantity ofthe second solvent acetic acid. The extracting solvent is introduced:into the vessel 4through a pipe 3 which communicates with the side ofthe vessel near the bottom. A drainpipe I is provided forr removing `thedepleted aqueous solution from the bottom of the vessel, and a pipe 5fastened to the top of the vessel is provided for withdrawing thesolvent after it has extracted the acetic acd. The vessel may beprovided with banles, such as staggered plates, perforated horizontalplates or bubble trays (not shown).

A conventional method of operating the' appay ratus of Figf'l is tointroduce a continuous stream of a 25% aqueous solution of acetic acidinto the vessel through the pipe 2, and a continuous stream of asolventmixture containing 25% ethyl acetate and 75% isopropyl ether through thepipe 3, while continuously draining oil' the depleted aqueous solutionthrough the drain pipe 4 and continuously withdrawing the solventmixture containing the extracted acetic acid through the pipe i at thetop of the vessel. The rate of ilow is so-ad- Y justed that the vesselis` always full.v For 4,ach

1 part of 25% aqueous acetic acid solution introduced it is customary tointroduce about 2.003 parts cf the solvent mixture composed of 25%kethyl acetate and 75% `isopropyl ether. Counte rcurrent and mutualpenetration between the Y solvent mixture and the aqueous acetic acidsolutionV is set up because the solvent mixture is lighter than thesolution.

My process, departs from the conventionalv practice jusvt described inseveral particulars, chief of which is that the extracting column ofliquid in the vessiis maintained with higher selectivity in one zone anda higherl gleanability in another zone, as shown'in Fig. 2.' This`permits the extracting column to extract acetic acid from aqueoussolution without extracting much water and at thesame-time to make asubstantially complete extraction of-`the acetic acid with a `minimum.Iamount of solvent. Asl 'indicated' previously, the action of theextracting column is analogous tothe action of a rectifying column Y indistillation.

If a mixture of two liquids having different boiling points is boiledand the resultingvapor is merely condensed, the bulk condensate will'-contain both liquid constituents of the mixture.

The simple process has` not exercised much selectivity as between thetwo` liquid constituent-s.Y

Howeven'if the vapor from the boiling mixture is passed through a reiiuxcolumn, the vreflux will tend'to carry back into the boiling mixture`that liquid constituent which condenses most' easilyV and the'condensate withdrawn from the upper end of the reflux column willcontain ahigher V.proportion of the other liquid ingredient. The

nearer one comes tothe top of the reflux column column (not shown)liquid -ingredient in the vapor. In other words, selectivity increasestoward the 'top of the column. However, as selectivity increases, the

gleanability of 'the rectifying column decreases' because any givenhorizontal section near the bottom of the column may contain a greaterquantity of the less easily condensible liquid than an equal section ata higher point in the column since vthe reflux, while containing only asmall proportion of the less easily condensible liquid. may amount toseveral times the vapor output at thetop of the column. The analogy withthe action of the extracting column employed in my process will beapparent.

Increased selectivity in one zone and increased gleanability in anotherzone of an extracting column of liquid may be obtained in several waysin the practice of my invention. Thus the process illustrated in Fig. lemploying a single extracting solvent may be improved by maintaining theliquiddn the bottomof the column hotterA thanh the liquid in the top ofthe column; Most solvents for acetic acid and the like manifest moregleanability when hot, i. e., a given volume of the solvent will holdmore acetic acid, but at the same. time the hot solvents have lessselec- Vtivity, i. e., they will pick up and hold a larger proportionofwater `along with the acetic acid.

`Hence maintaining the solvent in the bottom of the Yand gleanabilityfrom top to bottom ofthe column. The amount of gradation, of course,de.-`

pends upon the difference rin temperaturel between top and'bott'om ofthecolumn. The difference in temperature may be b rought about byaddfnghcat to the` solvent in the bottom of the column or by removingheat from the top.A

ing jackets (not shown) on the bottom of thek column or by 'coolingjackets on the top of the l It may also be accomplished by coolingliquids before they are fed into the top oi the column or by heatingliquids before theyiare fed -into the bottomoil the column.

Itis also possible to maintain an` extracting. column oiillquid with ahigh.v selectivityr in; onev zone and a high gleanability in another,zone with i gradually ,decreasing selectivity Aand graduallyincreasing`gleanabilityfrom the ilrstto .tli`ese[c.` I ond zone withoutAmaintaining a substantiaLditference in vthe temperature of the two zonesin the column. This is accomplished by employing two solvents, onehaving high selectivity, the j other high gleanability, and maintaininga high concentration of one solvent in one zone inthe? column and a highconcentration of the "other solvent in the otherl zone in the column.The

apparatus illustrated in Figs. 3 and 4 are adaptedr` to this practice. lv

Now considering the practice ofthe invention illustrated in Fig.' 3, itwill be seen that the extractor is somewhat similar to that of Fig. l,comprising an upright elongated cylindrical vessel G with closed endsand provided with an inlet pipe 1 at the side near the top forintroducing aqueous acetic acid solution, a drain pipe I at its lowerend for draining out the ydepleted soluaisance vent or solventmixturefof high gleanability. On from which the nal solvent product ofthe procthe side of the vessel intermediate the inlet for ess is drawn.

the fresh solutionand the inlet for the solvent of high gleanability isprovided a second inlet pipe II for introducing the solvent of highselectivity. The action of the extractor is improved if it contains aplurality of spaced baiiiegs, such as horizontal perforated trays orbubblev trays I IA.l

In a preferred lpractice of the invention the vessel is kept full anddilute acetic acid solution is pumped continuously into the vesselthrough its inlet'pipe whiledepleted solution is continuously removed.through the drain. For each part of acetic acid solution introducedthrough the pipe 1, 1.557 "parts of isopropyl ether are continuouslyintroduced into the vessel through the inlet pipev II and .474 part ofethyl acetate'is continuously introduced .through the inlet pipe IIJ.Through thertop outlet a continuous stream of solvent accompanied byextracted.,acetic'acidm is drawn. When the operation is conductedacording tothis method the* emciency of extraction is appreciablyincreased. That is, if the appa ratus illustrated in Fig. l and Fig. Y3are operated at the same rate with the same feed and with the sametemperature gradient from top to bottom of thevessel,l thedepletedsolution draining out of the apparatus of Fig. 3 will contain lessacetic acid than that draining out of the apparatus of Figd, otherthings being equal.. Moreover, other things being equal, the solventwithdrawn from the top of the apparatus of Fig. 3 will have ahighervconcentration of acetic acid and a lower concentrationof waterthan the solvent withdrawn from the top of the apparatus of Fig. 1.

These'improved results are attributable to the fact that -there is ahigh concentration of ethyl acetate in the vesselof Fig. 3 in theneighborhood of the inletp'ipe I0 .and a high concentration o! isopropylether in the neighborhood of the inlet pipe H. High concentration ofethyl acetate produces high gleanability. High concentration ofisopropyl ether produces high selectivity. The concentration of ethylacetate decreases in the vessel as the distance -from the inlet pipe Ilincreases. Similarly, the concentration of isopropyl ether increases inthe vessel as the distance from inlet pipe III4 increases. Consequently,there is established and maintained an extracting column in whichselectivity increases as gleanability decreases and vice versa.

The apparatus' illustrated in Fig. 4 is also adapted to the maintenanceof an extracting column of liquid with a gradually increasingselectivity and decreasing gleanability. The apparatus'comnrises twoupright elongated cylindrical vessels I2 and I1 with closed ends. Thevessel I2 is provided with an inlet I 3 on its side near the top forintroducing a dilute aqueous solution of acetic acid, and with a sideinlet Il near the bottom for introducing a solvent of high gleanability,say ethyl acetate. At the bottom of the vessel I2 is a drain I5 forwithdrawing the impoverished aqueous solution, and at the top of thevessel is connected an outlet pipe Il for withdrawing the solventproduct oi' the vessel l2, namely, a solution containing ethyl acetate,water and acetic acid.

The outlet pipe I6 is connected to the side of the second vessel I'Inear the top.l 'I'his second vessel is provided with a drain I9 at thebottom and with a side inlet I8 for a solvent of high selectivity, sayisopropyl ether, near the bottom. At the top of the second vessel is anoutlet 2l sayisopropyl ether.

Both vessels I2 and Il preferably are provided with a. plurality ofspaced transverse baffles such as perforated plates or bubble trays 2l,22.

The operation of the process in the apparatus of Eg. 4 is as follows:The rst extraction vessel I2 functions in accordance with prior artproposals. Dllute aqueous acetic acid is passed continuously into thecolumn through the inlet Il and passes downwardly to the drain I5 incountercurrent to aicontinuously rising stream of a solvent, sayethylacetate. The depleted aqueous solution is continuously drained out ofthe vessel at the lower end, and a new solution of ethyl acetate, aceticacid and considerable water is drawn o of the rst vessel andcontinuously introduced into the second vessel where it encounters acontinuous stream of a solvent of high selectivity, Water drops out ofsolution and is removed through the drain 'from the second vemel, andthe dewatered mixture is continuously' withdrawn from the top of thesecond vessel. n

It is in the second vessel that the column of liquid having a graduallyincreasing selectivity and a gradually decreasing gleanability ismaintained. In the zone where the pipe I4 enters the irst vessel theconcentration of ethyl acetate is highest and hence the gleanability ofthe column is ata maximum here. pipe Il in the vessel the concentrationof isopropyl ether is highest; hence this zone exhibits maximumselectivity. In between these two inlets there is a gradual decrease ingleanability from the entrance of I 4, and an accompanying increase inselectivity. The ethyl acetate in passing -through therst vessel picksup substantially all the acetic acid, but it also picks up more waterthan isopropyl ether will hold. YHence in the second column water isgradually thrown out of solution from the top of the column to thebottom and is removed through the drain. However, the effectivegleanability and the effective selectivity of 'the column of liquid inthe second vessel (on account of the lack of homogeneity of the solventcontent throughout the column) are both higher than would be the case ifthe iniiow from I6 were merely mixed indiscriminately with the inflowfrom I8.

' In vthe practical operation of the apparatus of Fig. 4, 0.785 part ofethyl acetate is pumped into the iirst vessel and 1.227 parts ofisopropyl ether are pumped into the second vessel for every l part of25aqueous acetic acid solution introduced into the ilrst vessel. 'Iheextraction which takes place in -the first vessel under these conditionsis equivalent to raising the acetic acid concentration from 25% to47.5%. At this higher concentration isopropyl Vether is more eiectiveand has in fact a higher gleanability as well as a higher 1 selectivity.

At the entrance of the On the 'group'of suitable solvents includesliquid hydro- -76 carbons, most of their halogen derivatives, most ofthe normal and mixed ethers and the liquid ethers boiling above 120 C; Y

Solvents having high gleanabilltyV for acetic acid in aqueous solutionsusually dissolve of water and are soluble in water up to The liquidesters boiling. below 100 C., the ethers boiling below 50 C.,'an`d-thehigh boiling ketones, aldehydes and alcohols are or"this'character.`

Following is a list ofsolvents which may be employed in the practice ofmy invention:

High selectivity High gleanability isopropyl ether. Ethyl etherlBenzene. toluene. xylene. Ethyl acetate. Pentane. Propyi acetate.Chloroform, carbon tetrachloride. Furlural. Metilene, ethylene,propylene Cycohexanones.

a i es.

' Octyl acetate. Propyl formate. Cyclohexanol acetate. Butyl alcohol.Tetralin, decalin. Cyclohexanol. Cyclohexane. Amyl alcohol. Ethylacetoacetate. Acetone oils-mixtures of prcpyl, Acetophenone. butyl.amyl, etc. Aryl ethers. Ketones. Carbon disullidel Methyl'propionate.

,Cresol (o, rn and p).

In choosing a pair of solvents for practicing the invention azeotroplcdistillation ratio, azeotropic boiling point and its proximity to theboiling point of the substance extracted should be borne ln mind if thesubstance extracted is to be separated from the extracting solvents bydistillation.

It will be understood that a column having a zone of high selectivityand another zone of high gleanabillty for the practice of the inventionmay be maintained by holding the zone of high glean-l ability at ahigher temperature than the zone of high selectivity. at the same timeemploying a different combination of solvents in the two zones ashereinbefore described.

It will be noted that in the column of Fig. 3 the gleanabillty increasestoward a lower `zone in the column and the selectivity increases towardan upper zone in the column, the direction of passage of theimpoverished solution (water) being downward. In the apparatus of Fig. 4the direction of passage of the impoverished solution (water) is alsodownward but the zone of high selectivity is more or less confined tovessel I1,

especially near inlet I8. This, however, I have found to be immaterial.,Both arrangements operate equally well.

I claim:

1. Process for extracting a substance from its ilrst solution in a rstsolvent which comprises maintaining a column of liquid containing one or.more other solvents for the substance which are at least partiallyimmiscible with the rst solvent, maintaining a relatively highselectivity and low gleanabillty for the substance in one zone of thecolumn of liquid, maintaining a graduall'y decreasing selectivity and agradually increasing gleanabillty furthe substance in successive crosssections ofv the column toward another zone therein. maintaining arelatively high gleanabillty and a relatively low selectivity for thesubstance in said other'zone of the column of liquid, said zone of highselectivity and said zone of high gleanabillty being maintained bymaintaining'a diierence in the temperature of the immiscible solvent inthe two zones, passing the substance to be extracted accompanied by therst solvent through the column of liquid and in contact therewith fromone zone to the other, withdrawing the resulting depleted ilrst solventfrom the column of liquid, and withdrawing from the column of liquid theresulting new solution of the substance in the immiscible solvent.

2. A process for extracting a substance from its first solution in afirst solvent which comprises maintaining a column of liquid containingone or more solvents for the substance which areat least partiallyimmiscible with the tirst solvent, maintaining a relatively highselectivity and low gleanabillty for the substance in one zonev of thecolumn of liquid, maintaining a gradually decreasing selectivity and agradually increasing gleanabillty for the substance in successive Vcrosssections in the column toward another zone therein, maintaining arelatively high gleanabillty-and a relatively low selectivity for thesubstance in said other zone of the column of liquid, heating, said zoneof high gleanability and low selectivity, passing the substance to beextracted accom' panied by the first solvent through the column ofliquid and in contact therewith from one zone to the other, withdrawingthe resulting depleted rst solvent from the column of liquid, andwithdrawing from the column of liquid the resultingv new solution of thesubstance in the partially irnmiscible solvent.

3. A process for extracting a substance from its ilrst solution in ailrst solvent which comprises maintaining a column of liquid containingone or more other solvents for the substance which are at leastpartially immiscible with the rst solvent, maintaining a relatively highselectivity and low gleanabillty for the substance in one zone of thecolumn of liquid, maintaining a gradually decreasing selectivity and agradually increasing gleanability for the substance ln successive crosssections in the column toward anotherzone therein, maintaining arelatively high gleanabillty and` a relatively low selectivity for thesubstance ln saidother zone oi' the column of liquid, removing heat fromthe column of liquid in the zone of high selectivity, passing thesubstanceto be extracted accompanied by the rst solvent through the co1-umn of liquid and in contact therewith from one zoneto the other,withdrawing the resulting depleted first solvent from the column ofliquid and withdrawing from the column of liquid the resulting newsolution of the substance in the lmmisclble solvent. y

JOHN WILLIAM AILQUIST.

